Loss of Smell and Taste in COVID Explained
New research awaiting peer review uncovers why the loss of sense of taste is one of the symptoms of COVID infection. New research has found that taste receptors have ACE2 and are also at risk for SARS-CoV-2 invasion.
Understanding the presence of viral infection in taste buds could help treat people with ‘long COVID’ who could continue to experience changes in or loss of taste months after the initial infection.
SARS-CoV-2 uses the angiotensin-converting enzyme 2 (ACE2) enzyme attached to the membranes of cells as their point of entry. ACE2 enzymes are present all over the body, especially in the lungs and nose, coinciding with COVID’s early symptoms of loss of smell and difficulty breathing. Loss of taste is another early COVID infection sign, although the mechanism behind this is unclear.
“By demonstrating the co-localization of SARS-CoV-2 virus, Type II taste cell marker, and the viral receptor ACE2, we show evidence for replication of this virus within taste buds that could account for acute taste changes during active COVID-19,” wrote the authors. “This work also shows that the proliferation of the taste stem cells in recovering patients may take weeks to return to their pre-COVID-19 state, providing a hypothesis for more chronic disruption of taste sensation, reports of which are now appearing in the medical literature.”
ACE2 is present on Type II taste bud cells on the tongue. There are three cranial nerves (CN VII, IX and X) that are involved in relaying taste information to the central nervous system. Taste is first discriminated in taste receptor cells (TRCs) within taste buds located in circumvallate (CVP), foliate (FLP) and fungiform papillae (FP) in the tongue. Three defined TRCs relay five basic tastes. Stem cells around the taste bud receive signals from taste cells, prompting differentiation into a replacement TRC.
The researchers identified 5000 to 10 000 taste buds, with almost half located at the base of the tongue called circumvallate papillae. ACE2 was found to be coexpressed with phospholipase C β2 used in the signaling of type II taste receptor cells. Taste receptor cells in the back of the tongue, a region known as fungiform pallipae, also had ACE2 receptors, providing further evidence of a viral entry point for SARS-CoV-2.
“Replication of virus can likely then occur undisturbed and allow for transmission from the taste bud into circulation, and locally infect lingual and salivary gland epithelium, oral mucosa and larynx and even on into the lungs,”
Case Studies of Altered Taste During and After COVID Infection
A 45-year old woman with COVID and controlled hypertension reported changes in her sense of taste, including not being able to taste the sweetness from chocolate and describing curry as ‘white’ and her tongue was enlarged and redder around the fusiform pallipae.
SARS-CoV-2 RNA was found in samples taken from that area, specifically in PLCB2 positive cells. The virus was also found in the lamina propria with disruptions in the stem cell layer. Symptoms improved after six weeks, along with taste perception.
A 63-year-old man with no preexisting conditions had donated samples of his fusiform pallipae in 2019, and more samples were taken six weeks after testing positive for COVID. He experienced several long COVID symptoms, including mild loss of taste — coffee tasted like mud, and he could not taste chocolate. The virus was not present in samples of his fusiform pallipae 10 weeks after infection. However, he had altered changes to the stem cell layer of the tongue compared to the 2019 samples.
The researchers suggested that stem cell impairments may affect taste bud cell turnover and could contribute to the delayed return of sense of taste.
Source: News-Medical.Net
Journal information: Doyle ME, et al. Human Taste Cells Express ACE2: a Portal for SARS-CoV-2 Infection. bioRxiv, 2021. doi: https://doi.org/10.1101/2021.04.21.440680
https://www.biorxiv.org/content/10.1101/2021.04.21.440680v1
